This disclosure relates to improved thermally conditioned (strengthened or tempered) glass, particularly glass sheets, and improved methods and apparatuses for the thermal strengthening of glass, particularly for glass sheets.
In thermal (or “physical”) strengthening of glass sheets, a glass sheet is heated to an elevated temperature above the glass transition temperature of the glass, then the surfaces of the sheet are rapidly cooled (“quenched”), while the inner regions of the sheet, insulated by the thickness and fairly low thermal conductivity of the glass, cool at a slower rate. This differential cooling produces a residual compressive stress in the glass surface regions, balanced by a residual tensile stress in the central regions of the glass. This is distinguished from chemical strengthening of glass, in which surface compressive stresses are generated by changing the chemical composition of the glass in regions nearer the surface, relative to the center, such as by ion diffusion. This also is distinguished from glass strengthening by combining or laminating together, while hot, layers of glass compositions having differing coefficients of thermal expansion, with lower expansion layers typically outermost, to result in surface compressive stresses upon return to ambient temperature. Relative to chemical strengthening and lamination, thermal strengthening processes are generally less expensive and much quicker to perform.
Thermally strengthened glass has advantages relative to unstrengthened glass. The surface compression of the strengthened glass provides greater resistance to fracture than unstrengthened glass. The increase in strength generally is proportional to the amount of surface compression. If a sheet possesses a sufficient level of thermal strengthening, relative to its thickness, then when and if the sheet is broken, it will divide into small fragments with dull edges rather than into large or elongated fragments with sharp edges. Glass that breaks into sufficiently small fragments, or “dices,” as defined by various established standards, may be known as safety glass, or “fully tempered” glass, or sometimes simply “tempered” glass.
Because the degree of strengthening depends on the temperature difference between the surface and center of the glass sheet, thinner glasses require higher cooling rates to achieve a given stress. Also, thinner glass generally requires higher final values of surface compressive stress and central tension to achieve dicing into small particles upon breaking. Accordingly, achieving full tempering (dicing) in glass with sheet thicknesses of around 3 mm or less has been exceedingly challenging if not impossible.